Rubber products, such as tires, are required to have excellent breaking resistance, wear resistance, crack growth resistance, and the like. Therefore, the use of natural rubber with excellent elasticity as the raw material for rubber products is known. In recent years, due to the decrease in rubber tree resources, the price of natural rubber has risen. Hence, synthetic rubber that has characteristics equivalent to those of natural rubber is necessary.
Natural rubber is substantially composed of polyisoprene having nearly 100% cis-1,4-bond content, and it is thought that the molecular structure of this polymer produces elasticity. Based on this knowledge of natural rubber, active research is being conducted on conjugated diene polymers having a high cis-1,4-bond content and on methods for manufacturing such conjugated diene polymers.
A method using a catalyst formed by a metallocene-type cationic complex of a gadolinium compound as a main catalyst is a known method for manufacturing a conjugated diene polymer having a high cis-1,4-bond content (see JP 2004-027179 A (PTL 1)). In the aforementioned method for manufacturing, a boron compound, an aluminoxane compound (in particular, PMMAO), or the like are also used as co-catalysts.